Line vs Mic amps for mixdown

[StarTrucker]

What is the design difference in a line amp vs a mic preamp, besides their intended function?

My reason for asking is because I am trying to understand the difference between amplifiers used in a console's mixflow for tracking vs. mixdown.
I understand the mic preamp for tracking, but if you had a simple mixer and switched all the mic xfrmrs for line input transformers, would that be all that was needed for remixing and mixdown purposes?

If I take something like an api312 and only replace the mic transformer (something like 600:50k) with a 10K:10K input Xfrmr, does it become a line amp that could be used for send/return duties during mixdown?

My concept of tracking and mixdown in this example is this:
Tracking :
Mic --> Mic input --> hardware compressors etc. --> converter --> computer

Mixdown
Computer --> Converter --> 10k:10k line amp --> compressors etc. --> converter --> computer

Anything besides the input transformer need to be changed?

 

[ruffrecords]

The main difference is rhe amount of gain they have. A mic pre needs to be capable of 60dB or more gain. A line amp rarely needs more than 10dB of gain.

A 50K:600 transformer has just under 20dB of gain. A 10K:10K transformer has 0dB of gain so you need to lose maybe 40dB of gain. However, a good mic pre will be able to alter its gain down to 20dB and in some cases right down to 0dB so you idea could work.

In the old days, they just used to switch in a pad (attenuator) at the mic input which simultaneously knocked down the level to one the mic pre could handle and also provided a 10K input impedance. A couple of 4K7 resistors and a 150 ohm resistor will do this ans drop the level by 36dB.

Cheers

Ian

 

[abbey road d enfer]

In addition to what Ian says, typical mixers need to have mic or line inputs.
In many cases, a different circuit will be used for line than for mic, but some use the same input circuitry by inserting a switchable pad for line level.
The latter usually doesn't have the same performance than the former, although some may argue that disposing of the separate line input allows putting more resources in the mic circuitry. It may be a favorable point when real estate is at a premium.
In practice, the difference is often negligible.
Now, when it comes to modifying an existing circuit, one must balance the cost of different options. Inserting a pad costs pennies, when replacing a xfmr costs pounds, dollars, rubles, roupies, shekel, whatever.

 

[sahib]

I have recently modified one of our 11 slot 500 racks' inputs for a user (in US) to be switched between mic and line inputs for exactly the same application. The line input path has U pads for around 40dB of attenuation.

A global SPDT toggle switch is used for turning on/off (micro) relays. Switch in one position the relays are off and +48V is present for mic inputs. Switch flipped in other position the relays are on for line inputs and the +48V is globally turned off to eliminate the risk of +48V being accidentally applied to the line outputs of the DAW.

You can probably do the same.

 

[Tubetec]

Isnt there a mod for Neve style input transformers that uses relays to flip the transformer back to front for line input ? ie its used as step down .

 

[JohnRoberts]

What is the design difference in a line amp vs a mic preamp, besides their intended function?

Besides more gain as Ian already noted, mic preamps generally present lower input impedance. Both mic and line inputs use bridging terminations, typically 10x the source impedance, but mics and line outputs present dramatically different source impedances (150 ohm to up to 1-2k).

My reason for asking is because I am trying to understand the difference between amplifiers used in a console's mixflow for tracking vs. mixdown.
I understand the mic preamp for tracking, but if you had a simple mixer and switched all the mic xfrmrs for line input transformers, would that be all that was needed for remixing and mixdown purposes?

as long as the paths don't saturate (clip), it shouldn't matter

If I take something like an api312 and only replace the mic transformer (something like 600:50k) with a 10K:10K input Xfrmr, does it become a line amp that could be used for send/return duties during mixdown?

IDK

JR

My concept of tracking and mixdown in this example is this:
Tracking :
Mic --> Mic input --> hardware compressors etc. --> converter --> computer

Mixdown
Computer --> Converter --> 10k:10k line amp --> compressors etc. --> converter --> computer

Anything besides the input transformer need to be changed?

 

[RoadrunnerOZ]

My reason for asking is because I am trying to understand the difference between amplifiers used in a console's mixflow for tracking vs. mixdown.
I understand the mic preamp for tracking, but if you had a simple mixer and switched all the mic xfrmrs for line input transformers, would that be all that was needed for remixing and mixdown purposes?

You could add a line transformer and tap into where the mic transformer output is and use a 4 pole switch to lift one and add the other and add an extra input socket to the front end of the line transformer - or use a single pole switch and maybe 2x 4pole relays lifting input and output of the mic transformer and switching in the line so you could use the same input socket. This would also isolate the 48V if you do the cut before the phantom feed resistors.

If I take something like an api312 and only replace the mic transformer (something like 600:50k) with a 10K:10K input Xfrmr, does it become a line amp that could be used for send/return duties during mixdown?

If the mic transformer output is 50K then in the above scenario you would just use a 10K:50K. Are you sure the mic transformer is 50K out?

 

[emrr]

Mixdown
Computer --> Converter --> 10k:10k line amp --> compressors etc. --> converter --> computer

2 thoughts.

The compressors ARE line amps. In the very old days when compressors first came along, it would be normal to replace the existing line amp with the new compressor amp. Several 1950's-60's modular consoles with plug in units have compressors that replace the line amps.

Modern system, don't need much gain.

Antique system, the preamps are lower max out, sometimes not more than 0dBm, the line amps (PGM = program) will be +24 to +30dBm max out with much higher gain. That's predicated on doing primarily speech or acoustic music work, if you're doing anything loud you don't need so much gain, but at least whatever is required to overcome passive mixing bus losses. Those will all be 150-600 ohm passive mix busses that need identical mic input transformers for matching.

 

[nielsk]

API 212/312/512 uses 600/150 to 10k
I have reversed this for line input and it works fine, if you have an output level control or attenuator (a must with API mic pre-amps) you can fine adjust but with the proper gain pot all the way down I get unity gain.
There will probably be some transformer saturation at high levels, so don't hit it with high levels of low frequency...

 

[Walter66]

Yes, Fairchild 670 gain was 7dB (no limiting). Just wondering why they needed that many tube stages for such a low gain?

 

[RoadrunnerOZ]

My reason for asking is because I am trying to understand the difference between amplifiers used in a console's mixflow for tracking vs. mixdown.
I understand the mic preamp for tracking, but if you had a simple mixer and switched all the mic xfrmrs for line input transformers, would that be all that was needed for remixing and mixdown purposes?

There are two types of basic console:
1 Mixing console which will have a mix of input types such as mic level low impedance high gain, line level high impedance low gain, instrument level very high impedance medium gain suitable for things like guitar or bass pickups - selection of input type and input socket (if separate) on each channel accessed by mic/line or mic/line/instrument switch. These are designed for mixing only and mix down to sub-groups and stereo. Some will have a line level direct out for each channel to enable live recording.
To use this type for recording and mixdown you need to repatch the inputs and each input needs to be a line level.
2 Recording console which has the same array of mic/line inputs and selector switches as above, then on top of that each channel will have line level direct out socket to tape/audio interface (tape send) and a line level return socket from tape/interface (tape return).
Some consoles called in-line had two faders which could swap identity - one large for send level to tape and one small for monitoring the returns while recording and a “flip” switch to allow the fader function to be swapped so the tape return would come to the large fader for mixdown. In these the EQ was usually switchable to allow to go in the record path or in the monitor path.
Some had a small rotary pot instead of a small slider fader.

In some consoles with dual input sockets the mic input was via a transformer and the line input was transformerless, others had transformers on both. Some have no transformers at all. Others have a single input combo socket with XLR for the mic and TRS for line feeding separate preamps or the same preamp with pads for line. Some have a single XLR with a switch for mic or line which introduces resistors in line position for padding the signal and presenting the correct impedance.

It should be easy enough for a trial to reverse the transformer input and output on a single channel and see if it works then use relays on each channel with a single switch to flip the transformers for all and disconnect 48V globally at the same time.

 

[ruffrecords]

My reason for asking is because I am trying to understand the difference between amplifiers used in a console's mixflow for tracking vs. mixdown.
I understand the mic preamp for tracking, but if you had a simple mixer and switched all the mic xfrmrs for line input transformers, would that be all that was needed for remixing and mixdown purposes?

Just to take this particular question one stage further, in many high end consoles, mic and line input were both used almost exclusively for tracking. For mixdown there were separate "tape return" inputs which could be switched in instead of the mic/line inputs. Some of the big Neve consoles had a huge relay called a T-bar relay that could switch 24 channels from mic/line to mixdown.

Cheers

Ian

 

[abbey road d enfer]

If the mic transformer output is 50K then in the above scenario you would just use a 10K:50K. Are you sure the mic transformer is 50K out?

A 10k:50k xfmr has a a gain of 7dB. Not adequate since it needs attenuation. Matching uimpedances is not the whole story.

 

[RoadrunnerOZ]

A 10k:50k xfmr has a a gain of 7dB. Not adequate since it needs attenuation. Matching uimpedances is not the whole story.

Agreed but the existing mic transformer is only giving 19dB of gain from mic level so how much gain would you need considering the line level coming into the line transformer at say +4dBu compared to around -40 for mic level? Surely there’s more than enough gain in the following stage?

 

[emrr]

Yes, Fairchild 670 gain was 7dB (no limiting). Just wondering why they needed that many tube stages for such a low gain?

Look again - it's a single stage amplifier so far as audio signal path is concerned.

 

[abbey road d enfer]

Agreed but the existing mic transformer is only giving 19dB of gain from mic level so how much gain would you need considering the line level coming into the line transformer at say +4dBu compared to around -40 for mic level? Surely there’s more than enough gain in the following stage?

Of course, so why put a step-up xfmr at the input? A step-down would be more appropriate.

 

[RoadrunnerOZ]

Of course, so why put a step-up xfmr at the input? A step-down would be more appropriate.

So maybe just reversing the transformer would work?

 

[abbey road d enfer]

So maybe just reversing the transformer would work?

That's a possibility. However, it would require a 4PDT switch, which may be an issue hen it comes to modding an existing circuit.

 

[ruffrecords]

So maybe just reversing the transformer would work?

Probably but there is more to it than that. So far the discussion has centred on managing the gain and input impedance. We have not talked about noise. More gain means more noise and mic pres tend to be optimised for lowest noise at the highest gains. Their noise performance is not always stellar at low gains. This is why in many consoles. the line input is switched in after the mic pre.

The question is what is the requirement? Is it just to provide and easy low cost way of switching an input from mic to line level and live with any compromises or is it to provide te best possible line input?

Cheers

Ian

 

[RoadrunnerOZ]

With the amount of gain reduction by reversing the 600:50K transformer you’d probably need to run the gain fairly high anyway - there’s always the pad switch, -20dB
Easy enough to just bypass the transformer and come direct into the next stage with a switch - pretty well what they used to do with the Neve consoles using a buffer stage to mix like in the 10884 - but there the line in from the patchbay is balanced coming into a 5532 then feeding the gain switch pot unbalanced and going into the buffer - here if the transformer is lifted out of circuit you’d have a problem as the circuit is likely unbalanced after the transformer output giving you unbalanced line in without inserting either a transformer or electronically balanced input. There’s also the problem of muting while switching no matter what method is used.